Internal combustion engines, for example, diesel engines, gasoline engines, or natural gas engines employ turbochargers to deliver compressed air for combustion in the engine. A turbocharger compresses air flowing into the engine, helping to force more air into the combustion chambers of the engine. The increased supply of air allows for increased fuel combustion in the combustion chambers of the engine, resulting in increased power output from the engine.
A typical turbocharger includes a shaft, a turbine wheel attached to one end of the shaft, a compressor impeller connected to the other end of the shaft, and bearings to support the shaft. The compressor impeller is often mounted to the shaft using a nut that engages threads on the shaft near a nose portion of the compressor impeller. The nut is tightened to push the compressor impeller onto the shaft and applies a clamp load on the compressor impeller. Some turbochargers employ a boreless impeller. In these turbochargers, the compressor impeller has a threaded portion and the compressor impeller is used as a nut, which engages the threads on the shaft. Such an assembly requires specialized tools to assemble the compressor impeller onto the shaft without damaging the blades on the compressor impeller.
Hot exhaust from the engine flows through the turbine housing and expands over the turbine wheel, rotating the turbine wheel and the shaft connected to the turbine wheel. The shaft in turn rotates the compressor impeller. As the temperature of the shaft increases because of heat transferred to the shaft from the hot exhaust, the shaft expands both diametrically and axially. In a typical embodiment where the shaft extends along the length of the compressor impeller, several factors can influence the clamp load on the compressor impeller. When the shaft increases in temperature beyond the surrounding oil temperature, the resultant increase in length also reduces the clamp load on the nose of the compressor impeller. Further during typical turbocharger operations the compressor impeller varies in length relative to the clamped portion of the shaft due to centrifugal load acting radially, because of the temperature variation due to variations in the intake air temperature variation, and because of compression of the air. Increased rotational speed shortens the impeller as does cold inlet air temperature. Heating due to compression of the intake air counteracts the reduction in the length of the compressor impeller. These changes in length affect the compressive force on the nose of the compressor impeller and therefore the mounting joint fixity at the mounting end opposite the contact area at the nose of the impeller. The other potential for distortion from the axis of rotation occurs at the nose interface. If the applied clamp load strays from the geometric center of the compressor impeller due to insufficient piloting of the shaft or if the reaction surface does not remain square to the central axis, a lateral component of the shaft, the clamp load can distort the impeller enough to impact balance of the rotor. This in turn may cause the impeller blades to contact the housing walls, causing damage to the impeller blades and/or to the housing walls.
U.S. Pat. No. 6,896,479 of Svihla et al. issued on May 24, 2005 (“the '479 patent”) discloses an impeller assembly that aims to simplify manufacturing and assembly. In particular, the '479 patent discloses a turbocharger rotor that has a turbine wheel and shaft, a compressor wheel and adapter and a separate thrust runner clamped together by a fastener rod. The '479 patent discloses that the adapter is piloted onto the shaft and the thrust runner is piloted on the adapter. The '479 patent also discloses that an anti-rotation coupling relates the phase angles of the shaft with the adapter and the thrust runner during assembly. In addition, the '479 patent discloses that a fastener rod extends through axial openings in the compressor wheel, the adapter, and the drive shaft and includes a threaded end, which engages with the turbine wheel. The '479 patent also discloses that a nut engaging with threads on the fastener rod clamps the compressor wheel, adapter, thrust runner, and drive shaft together.
Although the compressor wheel assembly disclosed in the '479 patent attempts to provide simplification of manufacture and assembly, the disclosed compressor wheel assembly may still be less than optimal. In particular, thermal expansion of the fastener rod and impeller during operation of the turbocharger of the '479 patent may contribute to variation in clamp load, which may impact overall rotor stability. The clamp load variation may cause the drive shaft and compressor wheel to rotate off center from an axis of rotation of the compressor wheel assembly. Additionally, in the event of catastrophic failure of the compressor wheel due to excessive speed or material fatigue, the aerodynamic load acting on the turbine wheel may propel the drive shaft, fastener rod, and the turbine wheel attached to the fastener rod out of the bearing housing.
The compressor impeller assembly of the present disclosure solves one or more of the problems set forth above and/or other problems of the prior art.